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NRPE is a common easy to use option for monitoring remote machines from Nagios or Icinga. Unfortunately NRPE, with **out of the box setup,* is not what I would consider secure.*

This tutorial looks at how you can secure your NRPE traffic by using NSClient++ both as a client and server (yes it runs on Linux as well) in conjunction with SSL certificates to provide certificate based authentication.

Target design

We are going to start by looking at what we want to achieve. What we want is:

  • Encrypted SSL data transfer
  • Secure client authentication
  • Secure server authentication

The solution I present here will use SSL and certificate based authentication to create a secure environment where we only allow trusted clients to communication with trusted servers. Unfortunately the solution requires a lot of infrastructure (configuration and administration) so it is probably not for everyone.

We will be using NSClient++ on both CLIENT and SERVER and connect the two over a secured SSL socket whilst using a central CA for authentication both ways.

This means your client can be sure it is a trusted server it is connecting to as well as the server can be assured it is a trusted client connecting.


I am by no means a security expert but from what I can tell this is a fairly secure model so hopefully it shall be safe enough for our monitoring needs (it is surely more secure the the default method).

So how do we achieve this then?

Well, lets start by adding certificates to the NSClient server to make sure the connection is encrypted.

Using certificates with NSClient++


The first goal on the agenda is to use certificates with NSClient++. In this case we are not looking for authentication instead only encryption so this will only help keep the data traffic hidden from prying eyes. to do this we first need to create a certificate.

I tend to use openssl for certificates not because it is good but because it works. I tend to consider openssl to be far to complicated for everyday use and always end up scripting things around it to make it easier to work with.

To create a certificate with openssl we can use the bundles perl script. In my case I start from scratch so I need to generate the CA using the following command (it will ask for CA password as well as some meta information):

Then we can request the first certificate and sign it:

All open ssl commands will ask for meta information as well as passphrases and passwords and the only thing which is important is the CA field (or common name) during the request. It has to match the hostname of the server ideally (if you want to enable host name validation) but lessens security down the line as you can steal certificates from another node as well as various man in the middle attacks. After running this we have three files in the folder:

-rwxrwxrwx 1 root root 3220 Dec  2 18:48 newcert.pem
-rwxrwxrwx 1 root root  963 Dec  2 18:47 newkey.pem
-rwxrwxrwx 1 root root  668 Dec  2 18:47 newreq.pem

The request (newreq) is not really useful anymore instead it is the certificate (newcert) and key (newkey) we are interested in. Before we copy these files over to our NSClient++ server we shall remove the passphrase from the key (unless you want to manually type a password whenever you start NSClient++).

openssl rsa -in newkey.pem -out newkey_open.pem

Then we rename and place these files under the under the security folder of the NSClient++ installation.

Next up we will configure the SERVER NSClient++ to use this certificate. Which is pretty straight forward first we enable the module (loading in all default values) then we set the relevant keys and finally we remove all defaults to keep the configuration clean and relevant.

del nsclient.ini
nscp settings --activate-module NRPEServer --add-defaults
nscp settings --path /settings/default --key "allowed ciphers" --set "ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH"
nscp settings --path /settings/default --key "certificate" --set security/server.pem
nscp settings --path /settings/default --key "certificate key" --set security/server\_key.pem
nscp settings --generate --remove-defaults
  • First we enabled strong ciphers and disable the unsecure ADH option by settings the *allowed ciphers* list to *"ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH"*
  • Then we set the *certificate* to *security/server.pem*.
  • Finally we set the *certificate key* to *security/server_key.pem*.

To test this we start NSClient++ in test mode and open up a new console where we will simulate a remote NRPE client connection.

nscp nrpe -- -H
I (0,4,1,66 2012-11-18) seem to be doing fine...

Here we don’t really see much in regards to our secure option but hopefully it is a bit less easy to eavesdrop on this connection since we have a proper certificate and not using the hard-coded DH key.

Next up we want to sprinkle some trust by using the CA to validate the certificates.

Enabling trust


The simplest way to enable trust when using SSL is to use a central CA when issuing the certificates. Since we already have the certificates in place for the SERVER all we need to do is copy the CA certificate over to the CLIENT and ask it to validate the remote server. The first thing we need to do is to copy over the cacert.pem file from the demoCA folder to the security folder of the CLIENT NSClient++ installation (I use the same for the SERVER for simplicity). Also note that I rename it to ca.pem for brevity. Next we start nscp in client mode again adding a set of options:

nscp nrpe -H --ca security/ca.pem --verify peer-cert --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
I (0,4,2,1 2012-08-08) seem to be doing fine...

The extra options are (in order):

  • --ca security/ca.pem The certificate authority to use (in our case the demoCA certificate)
  • --verify peer-cert What we want to verify (in this case that the peer is valid and has a valid certificate)
  • --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH As before for legacy reasons the default cipher list includes only unsecure ciphers so we need to change that.

If we quickly reconfigure NSClient++ without the certificate and retry we get the following result (which is not a very good error message, since it only says something is wrong):

nscp nrpe -H --ca security/ca.pem --verify peer-cert --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
error nrpe SSL handshake failed: short read
error nrpe Error: Failed to connect to: :short read
Error: Failed to connect to: :short read

Better trust?

Now it might not make too much sense for us to validate that the server we are calling has a valid certificate a better option would be to have the server validating incoming connections. We presumably know who we are calling so we can most likely trust the remote node after all. This is easily achieved but requires a bit more configuration as well as a bit more administration. This as we now are required to provide certificates for BOTH the CLIENT and the SERVER. so we need an extra set of certificates and we need to configure NSClient++ to use them.


So first off we need to go and generate some more certificates using the script again.

This time we copy the certificates over to the CLIENT side and rename them to client.pem and client_key.pem. For details refer to the section above also don’t forget to remove the passphrase from the key file.

Now we need to re-configure our SERVER to instead validate certificates which we as before do using the command line syntax. And as before we first add defaults to figure out what the key is inside the NSClient++ configuration file.

nscp settings --generate --add-defaults
nscp settings --path /settings/default --key ca --set security/ca.pem
nscp settings --path /settings/default --key "verify mode" --set "verify peer"
nscp settings --generate --remove-defaults

Then we restart NSClient++ SERVER and retry the connection command we issued before.

nscp nrpe -H --ca security/ca.pem --verify peer-cert --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
error nrpe SSL handshake failed: short read
error nrpe Error: Failed to connect to: :short read
Error: Failed to connect to: :short read

This is actually expected as we have not loaded the client certificate. Also since we are not interested (yet) in validating the client and thus use the CA certificate ca.pem file we remove all options we had relating to the CA before. Instead we add new ones for the CLIENT certificate as well as key.

nscp nrpe -H --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH --certificate security/client.pem --certificate-key security/client\_key.pem
I (0,4,2,1 2012-08-08) seem to be doing fine...

Going over the options we have:

  • --allowed-ciphers We still need to enable to strong chipers.
  • --certificate Our client certificate
  • --certificate-key The key for our certificate.

Complete trust

The last thing we are going to do is enable trust both ways. Client will trust server and server will trust client. Since we have already had trust both ways (just not at once) we only need to change the command to include the verification we had before again:


nscp nrpe -H --ca security/ca.pem --verify peer-cert --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH --certificate security/client.pem --certificate-key security/client\_key.pem
I (0,4,2,1 2012-08-08) seem to be doing fine...

As you can see we end up with a rather long command we can get around this by using some configuration but I will leave that for another day especially since the command will be templated in Nagios/Icinga anyway so the long command line wont be a problem. Lastly lets go over all the command line options one last time.

  • --ca security/ca.pem Set the CA certificate to use when validating remote peers.
  • --verify peer-cert Enable verification of peer certificates.
  • --allowed-ciphers ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH Enabled strong ciphers (and disable weak ones)
  • --certificate security/client.pem Set the client certificate
  • --certificate-key security/client_key.pem Set the client certificate key file.

And the configuration as well:

NRPEServer = enabled

allowed ciphers = ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
certificate = security/server.pem
certificate key = security/server\_key.pem
verify mode = peer-cert
  • ca = security/ca.pem Set the CA certificate to use when validating remote peers.
  • verify mode = peer-cert Enable verification of peer certificates.
  • allowed ciphers = ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH Enabled strong ciphers (and disable weak ones)
  • certificate = security/server.pem Set the server certificate
  • certificate key security/server_key.pem Set the server certificate key file.


So enabling security over NRPE is not that difficult using NSClient++. If it can be done using plain vanilla NRPE I am unsure of it is possible you can configure this using environment variables but I have never tried I am skeptical though.

The biggest drawback to using certificates for security is that you almost have to have infrastructure to manage it as manually generating and copying and configuring certificates is a lot of work. If you push your configuration out it could maybe be done using some publishing tool but my guess is you will need to manage this on your own.

I have some plans to provide a way to do this centrally and automatically using some NSClient++ scripting but haven't had time to do so yet. If you are interested in coming with feedback and helping out testing/developing such a solution please don’t hesitate to ask.

As always, feedback greatly appreciated (in all its forms)!

UPDATE: As Beaker pointed out the second command is -newreq not -newca again :)